The present invention relates to an imaging unit and to an image reading apparatus.
JP-A-2004-126284 discloses a contact image sensor module capable of clearly reading a translucent original held away from a surface of an original table by a holder or the like. In the contact image sensor module described in JP-A-2004-126284, a first rod lens array forms an optical image of a refection original on a light receiving surface of a first image sensor. A second rod lens array forms an optical image of a translucent original on a light receiving surface of a second image sensor. Thus, the contact image sensor module described in JP-A-2004-126284 can detect the reflection original, which is placed on the surface of the original table, and the translucent original disposed away from the surface of the original table.
However, to clearly form an optical image of the reflection original and an optical image of the translucent original on two image sensors, the contact image sensor module described in JP-A-2004-126284 needs to have two rod lens arrays, which differ in conjugate length from each other, and to provide a difference of elevation between the mounting surfaces of the two image sensors, which is used for adjusting focal positions. The conjugate length of a rod lens array is discretely determined according to a standard. This contact image sensor module has problems in that it is not easy to design a contact image sensor module, which clearly detects a reflection original and a translucent original, by selecting two kinds of rod lens arrays, which differ in conjugate length from each other, according to places, on which originals are placed, from standardized products, and that on the other hand, the manufacturing cost thereof increases in the case of using nonstandard rod lens arrays. Also, in the case of providing the difference of elevation between the mounting surfaces of the two image sensors to adjust the focal positions, this contact image sensor module has a problem in that the manufacturing cost increases due to complication of the structure thereof.
It is necessary to read image information recorded on a translucent original, such as a photographic film, at a resolution that is higher than a resolution at which image information recorded on a reflection original. However, each of the two image sensors described in JP-A-2004-126284 is what is called a one-channel image sensor. In the description, the “one-channel image sensor” means an image sensor constituted by a light receiving element having a single spectral sensitivity characteristic. Further, the multi-channel image sensor means an image sensor constituted by a plurality kinds of light receiving elements differing in spectral sensitivity characteristic from one another. Practically, for example, a three-channel image sensor is an image sensor including three kinds of light receiving elements respectively having red, green, and blue on-chip color filters. Therefore, an image reading apparatus having the contact image sensor module described in JP-A-2004-126284 detects pieces of image information respectively corresponding to channels, which are recorded on a translucent original, in a time sharing manner. In the case where image information corresponding to each channel, which is recorded on the original is read in a time sharing manner, a reading rate is low, as compared with a case where pieces of image information respectively corresponding to the channels are read in parallel. Thus, the image reading apparatus has a problem in that a read time is long. On the other hand, in the case where pieces of image information respectively corresponding to the channels are read in parallel, it is necessary to use image sensors having a large number of pixels, as compared with the case where the pieces of image information corresponding to the channels are read in a time sharing manner, and so that the manufacturing cost is increased.